1. Field of the Invention
The present invention pertains generally to the field of mechanical drive systems capable of moving a load back and forth between two end-of-travel positions such as for example, a door or gate actuating systems and more particularly relates to such a linear drive featuring a motor driven threaded drive shaft and a load pulling carrier assembly axially displaceable by rotation of the shaft. The novel drive is characterized by precise positive end-of-travel positioning of the load along the shaft by purely mechanical means irrespective of continued shaft rotation to thereby eliminate previously used timers and position sensors.
2. State of the Prior Art
Linear mechanical drives find wide application but are particularly in opening and closing doors and sliding gates such as remotely operated garage doors.
One particular application for screw drives of the type contemplated by this invention has been in the remote actuation of roll-up type truck loading doors on cargo van and trailer truck doors. Roll-up doors are made of several panels hinged together along their horizontal edges and held along their sides within slide tracks which are vertical along the door opening for holding the hinged panels in a flat vertical plane to close the door opening. The slide tracks curve to a horizontal position above the door opening such that pushing up on the door successively brings the panels to a horizontal out of the way position. Conventionally, such door actuating drives have included a worm gear or screw shaft mounted to the ceiling of the van/trailer cargo enclosure and driven by a reversible motor powered by the vehicle battery. A nut is threaded on the worm screw and is displaced axially by rotation of the screw. A load such as a van pull-up door is connected to and pulled by the nut between the opposite ends of the threaded drive shaft. This general type of linear drive is well known and widely used in many applications. Difficulties have been encountered, however, in applications requiring precise positioning of the load at one or both ends of the drive shaft. Rotary inertia of the drive motor introduces a positioning error in systems relying on timers or load position sensors to activate and deactivate the motor. More sophisticated systems capable of electronically sensing and accurately positioning the load are costly and require more complex installation wiring of the system. In many applications such as truck door and garage door actuators, it is desirable to minimize the cost and complexity of the system without, however, sacrificing reliability. A continuing need exists for simple drive systems capable of long term reliability and load positioning accuracy with minimal maintenance, particularly in difficult environments such as cargo compartments of transport vehicles where the drive system is exposed to severe vibration, shock, ambient temperature extremes, humididty and moisture.